Nowadays, a wireless transmission system in which signals are transmitted among a plurality of transmission apparatuses using wireless communication is widely used. The wireless transmission system does not require provision of fixed cables among the transmission apparatuses, and signals are transmitted using radio waves.
In recent years, in order to improve the transmission rate in a wireless transmission system, wireless signals are modulated by a multi-value modulation scheme such as 256 QAM (Quadrature Amplitude Modulation) and at high modulation speeds. However, because of the characteristic of the wireless transmission system, i.e., carrying out communication by causing wireless signals to propagate through the air, the wireless transmission system is susceptible to various factors in wireless transmission links, and therefore the communication quality tends to deteriorate. Accordingly, even when the aforementioned modulation scheme or the like is used, enormous overhead is required to carry out an error correcting process using error correcting codes and the like. Consequently, as compared to the wired transmission system via fixed cables, the wireless transmission system has a restriction in that it is difficult to widen the communication band.
As a measure to address the problem in widening the communication band, in the field of packet communication such as Ethernet (registered trademark), the link aggregation technique in which a plurality of physical links are bundled and can be used as one virtual link is defined as IEEE 802.3ad. Here, the physical link refers to the infrastructure of the physical layer that carries out transmission of electric signals in response to a service request from the data link layer. In other words, the physical link refers to a physical transmission path. According to the link aggregation, for example, a bundle of five physical links each being 100 Mbps can be used as a virtual link of 500 Mbps.